1. Field of the Invention
The present invention generally relates to a thermal printer for a portable data terminal, and more particularly to a thermal printer having a thermal printing head which is capable of printing an image on sheet material with rapidity despite the availability of a limited power source for driving the thermal printing head.
2. Description of the Related Art
A thermal printer employs heat in order to form an image on sheet material. There are two basic types of thermal printers depending on the particular sheet material employed. One type of thermal printer employs a thermal printing head having a plurality of thermal printing elements which selectively generate and apply heat to a heat sensitive paper to thereby form an image on the paper. Another type of thermal printer is one that employs similar thermal printing elements, however, an ink donor, such as an ink ribbon, is used for transferring ink to standard paper in response to heat generated by the thermal printing elements.
Printing speed for the above thermal printers depends in large part on the number of thermal printing or heat generating elements, and particularly on the number of these elements which can be driven at the same time. Since the development of semiconductor technology has made it possible to make a long thermal head including a large number of thermal printing or heat generating elements, it is important to drive as many of these elements at the same time as possible in order to reduce printing time. However, in order to drive these heat generating elements simultaneously, a battery with large power is required.
Thermal printers of the above type have several advantages, including small size and the ability to print quietly. Additionally, thermal printers which use heat sensitive paper are extremely small sized. As a result, the latter thermal printers can be employed as printing devices in portable apparatus, such as portable computers or data terminals or portable calculators. However, since small size and light weight is critical in a portable apparatus, the size of the battery is limited, which in turn limits battery power. Accordingly, since printing speed depends on how many thermal printing or heat generating elements can be driven at the same time, the limited power of the battery in prior art thermal printers prevents rapid printing.